This invention relates to the float process for forming flat glass in which molten glass is passed onto a pool of molten metal (usually consisting chiefly of molten tin) upon which the molten glass floats as it attains a smooth surface and is attenuated to the desired thickness. More particularly, the invention relates to improvements in delivering the molten glass from melting and refining means to the float-forming chamber.
In the float-forming process it has been recognized that the arrangement by which the molten glass is initially deposited onto the molten metal pool is critical for the sake of attaining the optical quality desired for flat glass. Glass that has been thoroughly refined and homogenized when passing through the canal or other vessel connecting the melting and refining apparatus with the forming chamber becomes slightly contaminated at least on its bottom portion by contact with the ceramic refractory materials from which the delivery structure is made. This contamination is known to produce distortion in the glass sheets produced from this glass. In early float processes, as typified by U.S. Pat. No. 3,220,816 (Pilkington), the molten glass was delivered onto the molten tin by means of a spout that produces a freely falling band of molten glass, a portion of which flows rearwardly and then outwardly upon initially encountering the molten metal. This flow pattern served the purpose of diverting the contaminated bottom surface portion of the molten glass stream to edge portions of the glass ribbon that subsequently formed in the float chamber. These edge portions could be trimmed away and discarded, and the central portion of the ribbon would be relatively free from refractory-induced distortion.
A different approach to this problem is disclosed in U.S. Pat. No. 3,843,346 (Edge et al.). There, only a surface portion of the molten glass is drawn from the melting furnace into the forming chamber, thereby avoiding refractory contaminated glass in all portions of the glass ribbon being made. However, even in that arrangement, a small amount of refractory contact is necessitated by the presence of a threshold member over which the glass flows immediately prior to contacting the molten metal. Although the threshold member can be made of relatively pure non-contaminating refractory material, its gradual erosion can contribute to some distortion defects in the glass, and it requires occasional replacement in order to maintain the desired standards of quality in the glass produced. Accordingly, it would be desirable to minimize refractory contact with molten glass as it is being delivered into the forming chamber.
In U.S. Pat. No. 3,843,344 (Galey), there is shown an arrangement in which the threshold is located upstream from the upper glass flow control member known as the tuile or "tweel." Such an arrangement may reduce the shear forces on the threshold and thus reduce erosion of the threshold, but, nevertheless, entails some refractory contact of the glass by the threshold as well as the tweel. A similar arrangement is shown in U.S. Pat. No. 4,395,272 (Kunkle et al.).
U.S. Pat. Nos. 3,468,649 (Delajarte et al.) and 3,765,857 (Lecourt) both show a cylindrical conduit delivering molten glass to a float-forming chamber. Neither patent discloses the material from which such a conduit could be fabricated nor any details of the construction or purpose of such a conduit. Since both of these patents deal primarily with other features, it appears that the conduit shown in each is merely an abstract schematic depiction of glass delivery means to a float-forming chamber.
U.S. Pat. Nos. 3,318,671 (Brichard et al.); 3,488,175 (Montgomery); and 3,679,389 (Kanai) each show forming a sheet of glass which is passed in a substantially vertical direction onto a molten metal bath. In order for the glass ribbon to be self-supporting in such an arrangement, the glass viscosity must be relatively high, and, therefore, significant smoothing or attenuation of the glass ribbon on the molten metal bath cannot be achieved unless the glass is significantly reheated while it is on the molten metal. Reheating detracts from the energy efficiency of the process. In U.S. Pat. No. 4,203,750 (Shay), a seemingly less viscous ribbon of glass is directed onto a molten metal bath where it is attenuated by means of edge rollers. Nevertheless, the viscosity of the glass would appear to be relatively high in order to permit shaping of the ribbon prior to delivery onto the bath, and the preliminary shaping of the ribbon entails significant refractory contact which may be undesirable from a contamination and optical distortion standpoint.
In U.S. Pat. No. 4,162,907 (Anderson), discrete gobs of molten glass are extruded onto a small bath of molten tin. Because such an arrangement is discontinuous its throughput rate is limited, and it disadvantageously depends upon a complex mechanism having many moving parts to deliver molten glass onto the molten metal.
It would be desirable if an arrangement could be provided for delivering molten glass continuously onto a molten metal pool for forming into flat glass by a float process that avoids the drawbacks of the prior art set forth above.